Gravatars are dynamically generated based on your registered email addresses. The MD5 hash of an email address is used as a unique ID for the avatar image associated with it.
The problem turned out to be that Gravatar converts emails to lower case, and so for the user accounts on your self hosted WordPress blog you must use lower case for user’s emails.
If you don’t, then you won’t get the right URI for your Gravatar’s images since the MD5 hash changes with the letter case.